A number of slipping property imparting agents for coating are presently known, including higher fatty-acid metal salt, higher fatty-acid amide, wax emulsion, polyethylene glycol, polyethylene glycol ester, liquid hydrocarbon oil emulsion, polyethylene dispersion and sulfurized fatty oil (Japanese Patent Application Laid-open No. 3-137295 and Japanese Patent Application Laid-open No. 58-8200). These slipping property imparting agents can be used in pigment coating materials of pH7 or higher that contain calcium carbonate. However, in clear coating materials of lower than pH7 whose main constituent is starch, these slipping property imparting agents have heretofore failed to satisfy the required slippage for clear coat, because they produce an agglutinated product called scum if the dispersion stability drops and a shearing load is applied during the coating process.
Against the backdrop of increasing use of recycled paper in paper production, the market is witnessing lower blending ratios of mechanical pulps or extracts from mechanical pulps. In addition, calcium carbonate fillers are being used to produce papers having a neutral pH level. As a result, the friction coefficient of printing paper is increasing in general. A higher friction coefficient of printing paper can lead to wrinkling or other problems during web offset printing that affect a smooth traveling of paper through the cylinders. Therefore, it is important to adjust the friction coefficient of paper to an optimal level in accordance with the specific operation of each printing plant.
To reduce the friction coefficient of paper, alkylketene dimer (AKD) or talc can be added to the material during the paper production process, or a slipping property imparting agent for pigment coat can be coated on the produced paper. If AKD is added to the paper material, in summertime AKD may migrate on the take-up roller, thereby causing the friction coefficient to decrease significantly on the top roller and consequently allowing the paper to slip during printing. When talc is added to the paper material, the blending ratios of white carbon filler, calcium carbonate filler or other fillers having a high specific scattering coefficient must be reduced, which inevitably reduces the opacity of paper. Therefore, these methods are not suitable in the production of high-quality papers. In addition, applying a clear pigment coat containing slipping property imparting agents can produce an agglutinated product called scum, if the dispersion stability drops and a shearing load is applied during the coating process, which can cause a major problem on the printing line.
The aim of the present invention is to provide a slipping property imparting agent for clear coat that reduces scum production even when the dispersion stability drops and a shearing load is applied during the coating process.